In recent years, along with rapid expansion of computers, it has been desired to reduce reflection on a display surface or to prevent electrification of the surface of CRT (cathode ray tubes) in order to improve the working environment of the terminal operators. Further, recently, it has been demanded to reduce the transmittance of the panel glass in order to improve the contrast, or to shield electromagnetic waves of extremely low frequencies which may adversely affect human bodies.
To respond to such demands, methods have been adopted such that (1) an electroconductive antireflection film is provided on the panel surface, (2) an electroconductive antireflection film is formed on the surface of a face plate of CRT etc., which is then bonded to a panel surface by a resin, and (3) a filter glass having an electroconductive antireflection film formed on each side is disposed in front of a cathode ray tube.
Among them, in the case of methods (2) and (3), it is common to form the antireflection film in a plurality of layers by a vacuum deposition method. A specific example of such a film construction may be the one disclosed in Japanese Unexamined Patent Publication No. 168102/1985. This publication discloses that an antireflection film is formed by a combination of a dielectric film with a low refractive index, a dielectric film with a high refractive index and an electroconductive film with a high refractive index. By coating a multilayer antireflection film having such a film construction on a panel surface, the visible reflectance of the surface can be reduced to 0.3% or less, and the surface resistance can be reduced to 1 k.OMEGA./.quadrature. or less. Further, the above-mentioned electromagnetic wave-shielding effect can thereby be imparted.
Further, as a method of increasing the contrast, it has been known that it is effective to use a light absorbing film as a part of its construction. For example, Japanese Unexamined Patent Publication No. 70701/1989 discloses a case wherein a stainless steel film having a film thickness of 4 nm, a titanium oxide film having a film thickness of 29 nm and a silica film having a film thickness of 95 nm were sequentially formed on a glass substrate by a vacuum vapor deposition method. By coating a multilayer absorptive antireflection film of this construction on a panel surface, the visible reflectance of the surface can be reduced to 0.3% or less, and the surface resistance can be reduced to 1 k.OMEGA./.quadrature. or less. Further, at the same time, the visible light transmittance can be reduced by a few tens %, whereby a high contrast can be attained.
On the other hand, method (1) include (a) a case wherein a panel is coated first, and then formed into a cathode ray tube, and (b) a case wherein a cathode ray tube is first formed and then surface coating is applied thereto. In either case, a so-called wet method such as spin coating is relied upon presently.
If a so-called dry method such as the above-mentioned vacuum vapor deposition method is used, in the case of (a), there is a problem such that due to the heat treatment in the step for forming a cathode ray tube after film forming, the film properties will be changed, and the desired performance can not be obtained. In the case of (b), it is necessary to set the entire cathode ray tube in a vacuum chamber. Accordingly, there will be restrictions in the volume and weight, and there is a problem that the handling is not easy.
A sputtering method as a typical film forming method of dry system has had a difficulty in high speed stable film formation of SiO.sub.2 which is a low refractive index material essential for the construction of an antireflection film. Therefore, in the sputtering method, no technology has been established for an industrial production of an antireflection film with a large area.
However, recently, due to the increasing demand for high levels of properties as mentioned above, the following problems have been pointed out for the surface treatment by a wet method. Namely, (1) in a wet method, control of the film thickness is difficult as compared with a dry method, and there is a difficulty in reproducibility or uniformity, when it comes to a multilayer film construction of at least three layers, which is desired for good antireflection performance, (2) the lower limit of the surface resistance so far attained by the wet method is about 10.sup.3 k.OMEGA./.quadrature., which may be adequate for antistatic purposes, but it is difficult to attain 1 k.OMEGA./.quadrature. which is required for shielding electromagnetic waves, and (3) it is difficult to impart absorptivity without impairing the antireflection performance.
On the other hand, the vapor deposition method has, in addition to the above-mentioned problem in the heat stability of the film properties, a problem that the film forming cost is substantially higher than the wet method, and it has been desired to develop an inexpensive film forming method.
Under these circumstances, various attempts have recently been made to develop a method for forming SiO.sub.2 stably at a high speed by sputtering. As a result, several methods are now being practically developed. For example, MMRS (metal mode reactive sputtering) as disclosed in U.S. Pat. No. 4,445,997 and C-Mag (cylindrical magnetron) as disclosed in U.S. Pat. No. 4,851,095 may be mentioned.
As a result, an antireflection film by sputtering is about to be realized. However, with respect to the construction of the antireflection film, the construction of a film which has heretofore been formed by vacuum vapor deposition is followed in many cases, and no film construction particularly effective by sputtering has been known.
The following constructions are known as conventional examples of antireflection films.
For example, J. D. Rancourt "Optical Thin Films User's Handbook" (McGRAW-HILL 1987) discloses at page 128 a spectral reflection curve in a case where a light absorbing film with a complex refractive index (n-ik)=2-i2 and a transparent film with n=1.65 are formed in thicknesses of 3 nm and 75.8 nm, respectively, in this order on a substrate with a refractive index of 2.35. However, in this case, presented are theoretical calculated values, and the reflection characteristics are explained as those corresponding to a so-called "V coat", where the reflection becomes 0 only with a single wavelength shown by a transparent double layer film which is a basic construction for antireflection. Thus, they do not represent low reflectance in a wide range wavelength region (such as from 500 to 650 nm).
Further, U.S. Pat. No. 5,091,244 discloses a case where a transition metal nitride film and a transparent film are formed in film thicknesses of from 6 to 9 nm and from 2 to 15 nm, respectively, sequentially from a substrate side, as a construction to reduce the reflection to incident light from the substrate side (i.e. incident light from the side opposite to the film surface side).
When a light absorbing film having a proper optical constant is formed in a thin thickness, the reflectance from the substrate side decrease, as disclosed, for example, in "Thin-Film Optical Filters", H. A. Macleod, MaGraw-Hill Publishing Co., 2nd Ed., pp65-66 (1989).
In the U.S. Patent, SiO.sub.2 is laminated in a thin thickness (from 2 to 15 nm).
However, this construction is designed for the purpose of reducing the reflection from the substrate side. In the case of a multilayered film containing a light absorbing film, the reflection is totally different as between the front and rear sides. Therefore, with this construction invented for the purpose of reducing the reflection from the substrate side, the reflectance from the film surface side is about 10% over the entire visible light region, whereby no reflection-reducing effect is obtained.
U.S. Pat. No. 5,091,244 discloses a four layer construction of glass/transition metal nitride/transparent film/transition metal nitride/transparent film, as a construction to reduce the reflection on the film surface side. However, the object is to reduce the visible light transmittance to 50% or less, and this object is attained by adding an another light absorbing layer and making the number of layers at least four layers, whereby there has been a practical problem from the viewpoint of the production cost.
As described in the foregoing, a film construction has not been known wherein a light absorbing film is contained as a constituting element, the film construction is basically a double layer construction, whereby the production cost is low, and it provides low reflectance within a wide range wavelength region to incident light from the film surface side.